DESCRIPTION (applicant's abstract): Virtually all organisms, including humans, utilize circadian clocks to regulate a wide variety of physiological and behavioral processes. The study of circadian rhythms in Drosophila provides a system for understanding neural pathways that link molecular changes in defined neurons with specific behavioral processes. Circadian clock components have been conserved from flies to humans and the mechanisms that are involved in circadian time keeping and clock resetting appear to share common themes. However, relatively little is known about the mechanisms of clock output. That is, how does the clock regulate circadian behavior and physiology? In this proposal, consideration will be given to understanding the mechanisms regulating the function of a Drosophila circadian clock output element called lark. Lark is the only well characterized clock output component in any animal species. Genetic and molecular studies indicate that the lark output element functions as a repressor to mediate the temporal regulation of adult eclosion (the emergence of adults from pupae). Lark is an evolutionarily conserved RNA-binding protein whose abundance oscillates in a clock-dependent manner in Drosophila. However, levels of lark RNA remain constant throughout the diurnal cycle indicating that the lark protein oscillation is due to a translational or post-translational mechanism. The studies proposed here will use variations of existing biochemical, molecular and genetic techniques to examine the circadian function of lark. Specifically, we will determine (1) the biochemical mechanism underlying the lark protein rhythm; (2) the effect of lark overexpression on eclosion; (3) if lark overexpression can affect the endogenous protein oscillation.